Bootstrapped gain stage questions

[hylaphone]

Hi Folks, I am toying around with the bootstrapped triode as shown here:https://www.valvewizard.co.uk/LessGainFromTubes.pdf
And also in Merlins preamp book, pg 89.I am using 12AX7, with a design goal of 12dB gain and 10Meg input impedance.I set it up as shown in fig. 1, with 100K R1, 1.5K Rk, 2.2M Rg and 22K R2.Gain is spot on a 12dB.I am trying to verify the input impedance by finding the -6dB point with series resistance as shown here:https://craiganderton.org/how-to-check-input-impedance/
Tried several times and it is consistently 1.7Meg. Am I missing something or is there an adjustment I should make in the circuit to reach 10Meg input Z?Thanks --

[tubeswell]

Doesn't sound like you are calculating input impedance correctly for the bootstrapped version(s). Bootstrapping gives 'apparent' increase in input impedance (because the signal present at the grid is not much different in amplitude from the signal present at the junction of the grid leak resistor with the cathode load)

[hylaphone]

Doesn't sound like you are calculating input impedance correctly for the bootstrapped version(s). Bootstrapping gives 'apparent' increase in input impedance (because the signal present at the grid is not much different in amplitude from the signal present at the junction of the grid leak resistor with the cathode load)

Yes I've seen reference the input Z "appearing" large, but is there a way to test that in the real world?

[pdf64]

....I am trying to verify the input impedance by finding the -6dB point with series resistance as shown here:https://craiganderton.org/how-to-check-input-impedance/
Tried several times and it is consistently 1.7Meg. Am I missing something or is there an adjustment I should make in the circuit to reach 10Meg input Z?

That method is no good when the meter resistance (eg 1M, 10M) will be anywhere near the circuit input impedance. 
I suggest to measure the output V AC instead.

[hylaphone]

That method is no good when the meter resistance (eg 1M, 10M) will be anywhere near the circuit input impedance. 
I suggest to measure the output V AC instead.

Doh! Forgot about that. This worked great, and lowering Rg to 1M made the -6dB point with 11Meg series resistance. Close enough for rock n roll, bebop, rockabilly, whatever.I appreciate the help.